Residual limb volume fluctuation is an important challenge for amputee prosthesis users. When the volume of the residual limb changes, the prosthesis fits differently. If the residual limb reduces in volume, as usually occurs over the course of a day for example, the limb becomes loose in the prosthetic socket, and stresses can concentrate in soft tissues over bony prominences, causing pain. The risk of limb injury is also increased. If the residual limb increases in volume in the socket, interstitial fluid pressure increases, potentially occluding blood flow through the residual limb. Tissues are denied nutrients, and restricted venous return can cause a buildup of cell waste products and deterioration of limb tissues. Both of these conditions can result in soft tissue injury. Accordingly, residual limb volume fluctuation has been recognized as a major challenge that should be a priority in prosthetics research. Given that residual limb breakdown occurs in as much as 24% to 41% of the amputee population at a time and that limb volume fluctuation is considered a principal cause of pain and tissue breakdown, efforts to understand and control limb volume change are clearly of major significance.
Part of the difficulty for practitioners in helping patients to manage limb volume fluctuation is the lack of a quick and quantitative means for assessment. Current practices for assessing volume change are slow and highly subjective. A practitioner typically asks a patient about limb pain and sock ply variation throughout the day, since, for example, as a residual limb reduces in volume during the day, the patient may add socks to the limb to improve its fit in the socket. That information is coupled with an understanding of the patient's pathology and an inspection of the residual limb. Tests that include the patient adding or removing socks during the day are used over a trial period. The clinician then makes an educated guess about what (if anything) needs to be done to the socket to improve the fit. During these efforts, the patient's limb is at risk.
Quantitative assessment should substantially speed up the process of diagnosing and deciding treatment for limb volume fluctuation, from weeks to minutes and allow insight early on in fitting the socket to the limb. Further, quantitative measurement should not only help in understanding the expected diurnal fluctuation of residual limb volume, but should also provide insight into its source.
Ideally, the measurement of volume change of the residual limb should continue throughout the day, as the patient engages in normal activities, since the effect of such activities on the volume of the limb can be important in assessing whether and how to modify a socket to achieve a better fit. The data relating to volume change should thus be recorded for an extended period, while the patient is mobile. To enable such mobility, the hardware that detects changes in the volume of the residual limb must be relatively compact and not interfere with the fit of the prosthetic socket on the residual limb.
One approach for measuring the volume of a residual limb is to monitor the bioimpedance of the limb over time. Several bioimpedance measurement products are commercially available; however, most of these are single frequency impedance measuring devices. Two products are multi-frequency impedance devices. Specifically, the ImpediMed™ device uses 256 frequencies, and the Xitron™ device uses 50 frequencies for sinusoidal current excitation in the range between 5 kHz and 1 MHz. While these devices are designed for total body analysis of extracellular fluid volume and total body fluid volume—and not for assessing the volume of a residual limb in a prosthetic socket—they can also support bioimpedance measurements on segmental regions of the body. The methods for determination of the fluid volume of a measured region, for all such instruments, are based on the Cole model, a stochastic model, a statistical model, or another modeling approach.
The assessment of the extracellular segmental volume of the lower residual limb of an amputee has unique challenges that none of these conventional bioimpedance measurement products can adequately address. The dynamic testing needed to understand the change in fluid volume of the residual limb requires a nearly real-time display of the changing fluid volume dynamics.
Currently, a Matlab™ software-based analysis of the Cole model, using the Xitron product as the bioimpedance measuring device, can provide a plot of extracellular fluid volume vs. time. At present, this method provides good experimental feedback for dynamic assessment of an amputee's changing residual limb volume, in near real time, when carried out in a clinical environment. Unfortunately, this method requires two computers, the Xitron device, and three engineers to operate and synchronize an experiment to achieve a usable result. Clearly, this conventional approach fails to meet the need for portability and lacks the capability to provide results in real-time, in a non-laboratory setting.
The problem that is experienced by amputees as the volume of their residual limb changes during the day and with activity is well-recognized. One approach that has been developed to address this issue to achieve a better fit as the volume of the residual limb changes (besides changing the number of socks on the residual limb) is to use a vacuum assist device (VAD), such as the ePulse™ system by Otto Bock. This vacuum assist device enables a patient to control a vacuum level applied to the socket cavity, which controls the force seating the residual limb within the prosthetic socket. Another approach that has been developed to address this issue of achieving a better fit as the volume of the residual limb changes is to adjust the volume of the socket and internal components. Examples include fluid-filled bladders (e.g., Active Contact System™, Simbex, Lebanon, N.H.; Volume Management Pads™, Ohio Willow Wood, Mount Sterling, Ohio), air-filled inserts (e.g., Pneu-Fit™, Prosthetic Concepts, Little Rock, Ark.; Pump-It-Up!™, Love Associates Inc, Batavia, N.Y.), and liners and sockets with electro-active, piezoelectric, or other types of “smart materials.” It would be desirable to automate the control of these and other volume management devices by monitoring the volumetric change of the residual limb in the socket, and thereby automatically maintain a better fit between the prosthetic socket and residual limb as the volume of the residual limb changes, particularly with changes in the activity of the patient. It might also prove beneficial to automate the control of other prosthesis design features as the residual limb changes volume, for example, the socket suspension, or the action of the prosthetic foot, since these features affect the force delivered to the residual limb, and thus, affect the fluid transport process in the residual limb. Currently, no source of a signal indicative of changes in the volume of a residual limb is available that might be used for this purpose.
Accordingly, it is evident that a new approach is needed, which integrates all the essential features required in a single compact system and in a form so that a non-engineer can use the system to reliably access the dynamic changes in the volume of an amputee's residual limb in real-time.
The enhanced understanding achieved through such volume change measurement should reduce tissue breakdown risks and improve the quality of life of individuals with amputated limbs.